B

B cell receptor: The antigen receptor in B cells, also called membrane immunoglobulin or membrane antibody.
B lymphocyte (B cell): A type of lymphocyte that develops to maturity in the bone marrow. After encountering antigen, B cells differentiate into antibody-secreting plasma cells, the effector cells on humoral immunity.
Backcross: A cross between a hybrid individual and one of the parental genotypes.
Background selection: The reduction in genetic diversity cause by selection against deleterious alleles at linked locus.
Bacteria: One of the three domains of life. These species have no nuclei and thus were group with archaea as ‘prokaryotes’ before rRNA sequences determine the three domains of life.
Bacterial artificial chromosome (BAC): An artificial version of a bacterial chromosome that can carry inserts of 100,000 to 500,000 base pairs.
Bacteriocin: A toxin produced by a bacterium that kills its competitors.
Bacteriophage: A virus that infect bacteria or archaea.
Bacteroids: A form of Rhizobium contained within the vesicles formed by the root cells of a root nodule.
Baculum: A bone that is contain in, and helps stiffen, the penis of rodents, raccoons, walruses, whales, and several other mammals.
Balance polymorphism: A stable polymorphism maintained by balancing selection. The ability of natural selection to maintain diversity in a population.
Balancer chromosome: A Drosophila chromosome that carries multiple inversions, recessive lethals, and dominant markers. Such chromosomes cannot recombine and cannot become homozygous and so can be used to keep individual wild-type chromosomes intact.
Balancing selection: Selection that maintains polymorphism.
Ballistospore: The actively discharged spore of basidiomycetes.
Bark: All tissues external to the vascular cambium, consisting mainly of the secondary phloem and layers of periderm.
Barr body: A dense object lying along the inside of the nuclear envelop in female mammalian cells, representing an inactivated X chromosome.
Barrier method: Contraception that relies on a physical barrier to block the passage of sperm. For example, condoms and diaphragms.
Bartholin’s glands: Glands near the vaginal opening in a human female that secrete lubricating fluid during sexual arousal.
Basal angiosperms: The most primitive lineages of flowering plants.
Basal body: A eukaryotic cell organelle consisting of a 9 + 0 arrangement of microtubule triplets, may organize the microtubule assembly of a cilium or flagellum. Structurally identical to a centriole.
Basal metabolic rate (BMR): The metabolic rate of a resting, fasting, and nonstressed endotherm.
Basal nuclei: A cluster of nuclei deep within the white matter of the cerebrum.
Base: A substance that reduces the hydrogen ion concentration of a solution.
Basement membrane: The floor of an epithelial membrane on which the basal cells rest.
Base-pair substitution:  A type of point mutation, the replacement of one nucleotide and its partner in the complimentary DNA by another pair of nucleotides.
Basidiocarp: The fruit body of basidiomycetes.
Basidium (-a): The terminal cell of a hypha that bears basidiospores.
Batesian mimicry: A palatable mimic evolves to resemble a distasteful model species and thereby suffers less predation.
Behavior: Everything an animal does and how it does it, including muscular activities such as chasing prey, certain nonmuscular processes such as secreting a hormone to attract mates, and learning.
Behavioral ecology: The scientific study of animal behavior, including how it is controlled, how it develops, and contributes to survival and reproductive success.
Benign tumor: A mass of abnormal cells that remain at the site of origin.
Benthic zone: The bottom surface of an aquatic environment.
Benthic: Living at the bottom of a body of water.
Beta-oxidation: A metabolic sequence that breaks fatty acids down to two-carbon fragments that enter the citric acid cycle as Acetyl CoA.
β-sheet: Common structural motif of proteins in which linear amino acid sequences (strands) located in different regions of the polypeptide chain align adjacent to each other and are stabilized by hydrogen bonding between backbone atoms located in different strands.
Biennial: A flowering plant the requires two years to complete its life cycle.
Big-bang reproduction: Also called semelparity.  A life history in which adults have but a single reproductive opportunity to produce large number of offspring. For example the life history of Pacific salmon
Bilaterian: Member of the group that includes the majority of animal phyla and include all animals with bilateral (right/left) symmetry.
Bile: A mixture of substances that is produced in the liver, stored in the gallbladder, and acts as a detergent to aid in the digestion and absorption of fats.
Binary fission: The type of cell division by which prokaryotes reproduce. Each dividing cells receive a copy of the single parental chromosome.
Binomial nomenclature: Naming scheme for species in which there is a genus name and a species name.
Biological species concept (BSC): Definition of species as groups of individuals that can successfully interbreed with each other in nature but that are reproductively isolated from other such groups.
Biomass: The mass of living material.
Biometry: The application of statistical methods to biology.
Biodiversity: the variety of animals, plants, and other organisms found in a particular place like Earth.
Biodiversity hotspot:  A relatively small area with an exceptional concentration of endemic species and a large number of endangered and threatened species.
Bioenergetics: The flow of energy through an animal taking into account the energy stored in the food it consumes, the energy used for basic functions, activity, growth, reproduction, and regulation, and the energy lost to the environment as heat or in waste.
Biofilm: A surface-coating colony of prokaryotes that engage in metabolic cooperation.
Biogenic amine: A neurotransmitter derived from an amino acid.
Biochemical cycle: Any of the various nutrient circuits, which involve both biotic and abiotic components of ecosystems.
Biogeography: The study of the past and present distribution of species.
Bioinformatics: Using computer power, software, and mathematical models to process and integrate biological information from large data sets.
Biological augmentation: An approach to restoration ecology that uses organisms to add essential materials to a degraded ecosystem.
Biological clock: An internal timekeeper that controls an organism’s biological rhythms. The biological clock marks time with or without environmental cues.
Biological magnification: A trophic process in which retained substances become more concentrated with each link in the food chain.
Biological species concept (BSC): Definition of a species as a population or groups of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring, but not able to produce viable, fertile offspring with members of other populations.
Biology: The study of life.
Biomanipulation: A technique for restoring eutrophic lakes that reduces populations of algae by manipulating the higher level consumers in the community rather than by changing nutrients levels or adding chemical treatments.
Biomass: The dry weight of organic matter comprising a group of organisms in a particular habitat.
Biome: Any of the world’s major ecosystems, classified according to the predominant vegetation and characterized by adaptations of organisms to that particular environment.
Bioremediation: The use of living organisms to detoxify and restore polluted and degraded ecosystems.
Biosphere: The entire portion of Earth inhabited by life, the sum of all the planet’s ecosystems.
Biota: All organisms that are part of an ecosystem.
Biotechnology: The manipulation of living organisms or their components to produce useful products.
Biotic: Pertaining to the living organisms in the environment.
Biotroph: Fungus deriving its nutrients from living cells of a host.
Bipolar cell: A neuron that synapses with the axon of a rod or cone in the retina of the eye.
Bipolar disorder: Depressive mental illness characterized by swings of mood from high to low, also called maniac-depressive disorder.
Birth control pills: Chemical contraceptives that inhibits ovulation, retard follicular development, or alter a woman’s cervical mucus to prevent sperm from entering the uterus.
Blade: A leaflike structure of a seaweed that provides most of the surface area for photosynthesis. Also, the flattened portion of a typical leaf.
Blastocoel: The fluid-filled cavity that forms in the center of the blastula embryo.
Blastocyst: Stage in mammalian preimplantation development immediately before implantation that contains cells arranged in a ball with a central cavity.
Blastoderm: An embryonic cap of dividing cells resting on a large undivided yolk.
Blastomere: After fertilization, the zygote undergoes multiple rounds of cleavage. The cells of the cleavage stage embryo are termed blastomeres.
Blastopore: The opening of the archenteron in the gastrula that develops into the mouth in protostomes and the anus in deuterostomes.
Blastula: The hollow ball of cells marking the end stage of cleavage during early embryonic development.
Blebs: Protrusions arising from a local disruption of plasma membrane-actin cortex interaction. they determine amoeboid locomotion from amoeba to vertebrates and serve as the means to explore the environment during development, immune surveillance, and cancer.
Blood: A type of connective tissue with a fluid matrix called plasma in which blood cells are suspended.
Blood pressure: The hydrostatic force that blood exerts against the wall of a vessel.
Blood vessel: A set of tubes through which the blood moves through the body.
Blood-brain barrier: A specialized capillary arrangement  in the brain that restricts the passage of most substances into the brain and prevent dramatic fluctuations in the brain’s environment.
Blue-light photoreceptors: A class of light receptors in plants. Blue light initiates responses such as phototropism and hypocotyl elongation.
Body cavity: A fluid-containing space between the digestive tract and the body wall.
Body plan: An organism overall morphology that is created by the reproducible spatial positioning of differentiated cell types.
Bohr shift: A lowering of the affinity of hemoglobin for oxygen caused by a drop in pH; facilitates the release of oxygen from hemoglobin in the vicinity of active  tissues.
Bolus: A lubricated ball of chewed food.
Bone: A type of connective tissue, consisting of living cells held in a living matrix of collagen fibers embedded in calcium salts.
Book lung: An organ of gas exchange in spiders, consisting of stacked plates contained in an internal chamber.
Bottleneck effect: Genetic drift resulting from the reduction of a population, typically by a natural disaster, such that the surviving population is no longer genetically representative of the original population.
Bottom-up model: A model of community organization in which mineral nutrients control community organization because nutrients control plant numbers, which in turn control herbivore numbers, which in turn control predator numbers.
Bowman’s capsule: A cup-shaped receptacle in the vertebrate kidney that is the initial, expanded segment of the nephron where filtrate enters from the blood.
Brachiopod: Member of the phylum brachiopoda within the lophotrochozoa, composed of a group of marine animals that superficially resemble clams but that are only distantly related to mollusks.
Brain hormone: A hormone produced by neurosecretory cells in the insect brain, that promotes development by stimulating the prothoracic glands to secrete ecdysone.
Brainstem: Collection of structures in the adult brain, including the midbrain, the pons, and the medulla oblongata; functions in homeostasis, coordination of movement, and conduction of information to higher brain centers.
Branch: Portion of the evolutionary tree diagram connecting two nodes.
Branch length: The length of a particular branch in an evolutionary tree and is used to represent the amount of evolutionary change or time.
Brassinosteroids: Steroid hormones in plants that have a variety of effects including cell elongation, retarding leaf abscission, and promoting xylem differentiation.
Breathing: The process involving alternate inhalation and exhalation of air that ventilates the lungs.
Breathing control center: A brain center that directs the activity of organs involved in breathing.
Breeding value: The sum of the average effect of each gene.
Bronchiole: One of the fine branches of the bronchus that transport air to alveoli.
Bronchus: One of a pair of breathing tubes that branch from the trachea into the lungs.
Brown alga: A phaetophyte; a marine, multicellular, autotroph protist that is the most common type of seaweed. Include kelps.
Brown fat: A tissue in some animals, located in the neck and between the shoulders, that is specialized for rapid heat production.
Browse: to eat or nibble or feed on (leaves, tender shoots or other soft vegetation), to graze.
Bryophyte: A moss, liverwort, or hornwort; a nonvascular plant that inhabits the land but lacks many oft he terrestrial adaptations of vascular plants.
Bryozoan: Member of a phylum (Ectoprocta or Bryozoa) of sessile colonial animals, that are superficially similar to corals but are instead members of the Lophotrocozoa.
Budding: An asexual means of propagation in which outgrowths from the parent form and pinch off to live independently or else remain attached to eventually form extensive colonies.
Buffer: A substance that consist of acid and base in a solution and that minimizes changes in pH when extraneous acids or bases are added to the solution.
Bulk feeder: An animal that eats relatively large pieces of food.
Bulk flow: The movement of water due to a difference in pressure between two locations.
Bundle sheath: A protective covering around a leaf vein, consisting of one or more cell layers, usually parenchyma.
Bundle-sheat cell: A type of photosynthetic cell arranged into tightly packed sheaths around the veins of a leaf.